Humming Beethoven's Ninth without certainty (5)

I believe the answer is:

niffy

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'humming' is the definition.
(informal term meaning smelly)

'beethoven's ninth without certainty' is the wordplay.
'beethoven's ninth' becomes 'n' (9th letter of 'beethoven').
'without certainty' becomes 'iffy' (I can't explain this - if you can you should believe this answer much more).
'n'+'iffy'='NIFFY'

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(Other definitions for niffy that I've seen before include "Smelly (sl.)" , "stink" , "Smelly (slang)" .)

I've seen this clue in The Times.

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